This invention relates to a container and a thermo-molding apparatus and a thermo-molding method for molding such a container. More particularly, the present invention relates to a legged and bottomed cylindrical thin wall container or a bottomed cylindrical container having the bottom wall connected to the inner barrel of the body section produced by folding the latter back at the grounding edge of the container, which is molded by thermo-molding and made free from any gap in the inside of the junction of the leg or the body inner barrel and the body wall, and also a thermo-molding apparatus and a thermo-molding method for molding such a container.
Legged and bottomed containers molded by thermo-molding a thermoplastic resin sheet and made free from any gap in the inside at the site corresponding to the leg shaped by bending the sheet and thermo-molding apparatus and thermo-molding methods for molding such bottomed containers are known to date. For example, Japanese Patent Laid-open Nos. 2000-225642 and 2001-001395 disclose such bottomed containers and thermo-molding apparatus and thermo-molding methods for molding such bottomed containers.
According to the above patent documents describing the prior art, a bottomed container is thermo-molded either by using a thermo-molding apparatus comprising an upper plug unit having a pressure molding plug and cooling male mold, and a unit of female mold including an upper mold for forming the mouth section and the body peripheral section of the container, an intermediate mold for forming the lower part of the body peripheral section and the outer peripheral surface of the bottom rim (or foot) and a bottom mold for forming the inner peripheral surface of the bottom rim and the bottom wall or by using a thermo-molding apparatus comprising a pressure molding plug, a female mold and a bottom mold.
More specifically, a container is molded by pushing the pressure molding plug into the female mold to shape the container so as to make it show the profile of the surface of the female mold, while holding the bottom mold to the lowered position, and subsequently raising the bottom mold to form the bottom wall and the bottom rim respectively on the upper surface of the bottom mold and between the bottom mold and the female mold.
The bottom rim is made either to flare toward the lower end and show a triangular cross section or to show a cylindrical profile that is vertically extending with a same diameter.
With the prior art, regardless if the bottom rim is flared or made to show a vertically extending cylindrical profile, the difference between the outer diameter of the upper end of the mold surface of the female mold (more particularly its intermediate mold) for forming the outer peripheral surface of the bottom rim and the outer diameter of the mold surface of the bottom mold for forming the inner peripheral surface of the bottom rim is greater than the thickness of the resin sheet and slightly smaller than the twice of the thickness because the bottom rim is formed between the mold surface of the mold surface of the female mold (more particularly its intermediate mold) and that of the bottom mold.
Therefore, if twice of the resin sheet that is used for producing the container is smaller than the difference of the diameters, a gap is produced along the inner surface of the container at the site that corresponds to the bottom rim.
This means that, with either of the above described thermo-molding apparatus and the corresponding molding method, it is not feasible to use resin sheets having different thicknesses and particularly thin resin sheets cannot be used.
If thin resin sheets are to be used, the difference between the outer diameter of the upper end of the mold surface of the female mold (more particularly its intermediate mold) for forming the outer peripheral surface of the bottom rim and the outer diameter of the mold surface of the bottom mold for forming the inner peripheral surface of the bottom rim needs to be reduced.
Additionally, the thickness of resin sheet can show variances in the stages before forming the bottom rim because the thickness of the original roll, the thermo-molding temperature, the degree of vacuum and the timing of applying compressed air can vary significantly. Therefore, it is very difficult to constantly maintain the molding conditions that make the thickness of the bent resin sheet slightly greater than the gap between the metal molds.
Thus, while the above described problem may be relatively insignificant when molding a large container by using a thick resin sheet, the thickness may have to be more rigorously controlled when a relatively thin resin sheet is used. In other words, the prior art is not suited for molding small containers.